Light-emitting diodes (LED) lighting has become increasingly more popular in recent years. As a source of light, LED has many advantages over incandescent lighting, including high efficiency, reliability, and long life. However, the brightness of LED depends on temperature P-N junction and ambient temperature fluctuations. This imperfection limits the use of LED in some areas of applications.
There are many patents which describe methods to address LED brightness, attempting to correct operating conditions to create stable lighting brightness. Most of these methods use external temperature sensors, such as PTC and NTC thermo-resistors, semiconductor diodes, and transistors of various sorts.
All circuits that use external temperature sensors have one shortcoming: they assume that the temperature of an LED is the same as the temperature of the sensor. This assumption is not valid; the temperatures of the LED and the sensor are not the same. Because of this, the temperature brightness compensation of LED with external temperature sensors is far from inexact. Moreover, if ambient temperature varies quickly, compensation of brightness occurs after the circuitry reaches thermal equilibrium. Additionally, the added external sensors and components reduce reliability and increases the costs of illuminators.
Currently, the following LED drivers integrated circuits (ics) include temperature brightness compensation:
A) Driver CL25 is a two-terminal element that functions as a current stabilizer. A typical application for the CL25 is to drive LEDs with constant current of 25 ma. It can also be used in parallel to provide higher current such as 50 ma, 75 ma or 100 ma. Typical temperature coefficient about +0.01%/° C. is insufficient for brightness compensation for most LEDs.
B) Driver MT7910 accepts a wide ranging dc voltage input to a switching regulator and used to power high brightness LEDs, and can be tuned to supply a wide range of current from just a few milliamps to more than an amp. It also includes a 0-245 mv linear dimming and temperature compensation of the LED current. For proper operation, this IC needs an external temperature sensor and a few ancillary components, such as a power transistor, damping diode, inductor, and capacitor.
The necessity of an external temperature sensor is a significant limitation in use of LED drivers.
Because an LED is a semiconductor diode, its forward voltage decreases when temperature rises and increases when temperature drops; this realization allows an LED to be used as a temperature sensor. Patent U.S. Pat. No. 7,683,864 “LED driving apparatus with temperature compensation function” describes the following method: the electric circuit measures the forward voltage of an LED, compares it with a reference voltage and then manages current flow—increasing current when the voltage difference indicates a temperature rise and decreasing current when the temperature drops.
FIG. 1A is a block diagram of a prior art LED driving unit. Reference to FIG. 1A, the prior art's LED driving unit includes: reference voltage generator 100, the amplification unit 200, driving unit 300, LED 400, repeating amplifier 500, and current limiter 600. The forward voltage across LED 400 is fed into the pass repeating amplifier 500 on the amplification unit 200 where is compared with reference voltage generator 100. Difference signal pass current limiter 600 and come to driving unit 300. A driving unit 300 adjusts a supply voltage of LED 400 in response to the voltage of the differential amplification unit 200. An increased temperature causes a decrease of the forward voltage across LED 400. A difference voltage between the reference voltage generator 100 and a forward voltage to LED 400 is increased and force driving unit 300 increases LED supply current.
The prior-described prior art has difficulties. The forward voltage across the LED depends not only on temperature but also on forward current. An increase of forward current with increasing temperature causes an additional increase in forward voltage. The control system perceives this fact as decreasing of temperature and stops correcting the brightness. Therefore the compensation of brightness is partial, as confirmed by the experimental evidence shown on FIG. 1B from the aforementioned patent.
The temperature dependence of brightness is determined by the type of semi-conductive material. For example, ultra-bright LEDs TLCX510 of Company Vishay are made of material aluminium indium gallium phosphide on gallium arsenide to radiate light of different color. Each color LED has different operating characteristics, including temperature dependence on brightness.
The brightness of an LED can be reported as a comparison relative to its reference brightness level, typically at 25° C. By definition, the relative brightness of an LED at +25° C. is one; in the range of temperatures −40° C. to +85° C., the change of relative brightness is 2.2 to 0.6 for red LEDs, 2.3 to 0.4 for yellow LEDs, and 2.6 to 0.48 for green.
Other materials show similar changes in brightness. For example, ultra bright LEDs type TLHB580 made of gallium nitride on silicon carbide can radiate blue or white (with phosphor) light. As temperature ranges from −10° C. to +100° C., brightness changes 1.15 to 0.3.
The LED industry is continuously in need of better ways of providing consistent brightness in LEDs over a range of temperature and operating conditions that does not depend on external sensors whose operational characteristics are susceptible to temperature change.